Review: R9M-Lite Module & Latency Testing vs TBS Crossfire

Here is an overview of the Frsky R9M-Lite module, and we will test the latency and see how it compares to the TBS Crossfire. Both are great 900MHz RC systems for long range and many are hesitating which one to get. With latency being one of the main considerations we hope you find the test useful.

We previously reviewed the R9M module, which is for the radios with JR module bay such as the Taranis X9D-Plus, Q X7 and Horus X10S. The R9M-Lite module is the miniature version which is designed specifically for the Taranis X-Lite radio.

Where To Buy?

What is the R9M-Lite Module for?

The R9M-Lite is an affordable long range option compared to the Crossfire. It’s easy to install on the X-lite without doing any mods, basically just plug and play! The X-Lite actually support Crossfire too but it requires special mounts.

900MHz radio system provides a much more solid and reliable link compared to the built-in 2.4GHz RF module in the X-Lite. You can fly around big fields and not have to worry about losing signal. You might even be able to go behind obstacles you could never dream of with 2.4GHz.

Radio receiver options are also pretty good, the R9-mini and R9MM receivers are even smaller than the 2.4GHz R-XSR, even better at half the price! But just like any 900MHz systems, antenna mounting is different than 2.4GHz. You need to mount them in a specific way to have the best possible signal.

Installation and Setup

The way we hold the transmitter will have the antenna pointing directly at the aircraft which gives a weak signal. You might want to get a 45 degree or even a right-angle RP-SMA adapter.

Flash the correct firmware on your radio and RF module (and receiver), and select the correct mode for your region. Using the correct frequency for your region is critical to signal reliability.

Here is a screenshot of the Model Setup in OpenTX.

Latency in our Radio Control System

The latency test was done by Andrey Mironov, with his permission to share the result here. You can learn more about the TBS Crossfire in this article. Check out our review of the Crossfire vs. R9M.

The latency in our RC link can be divided into 4 parts:

Gimbals => TX Interface (OpenTX) => TX module => RX => FC

The latency is measured between the points before OpenTX and after RX (bold text).

Measuring Latency in R9M-Lite and Crossfire

The latency are measured with the TX modules installed in the Frsky X-Lite. Other gear used:

  • R9M-Lite TX module with R9 Mini receiver
  • Crossfire Micro TX module with Crossfire Micro V2 receiver

Further Reading: Does the Frsky X-Lite support Crossfire?

Note that there is a stick filtering (6-sample averaging) that exists in OpenTX, which introduces a varying delay. To properly measure latency a custom version of OpenTX was built to get rid of this filtering. This was explained in more detail in our previous latency test of the X-Lite.

First test consisted of tapping into the gimbal output line and generating a 260Hz PWM signal with a STM Discovery board, altering between 10% and 90% duty cycle every 128 milliseconds, while observing the resulting receiver output signal frame over the course of 60 seconds.

Latency is measured between the falling edge of gimbal PWM signal and the start of corresponding RX signal frame.

Note that this doesn’t take RX signal frame length into account. The frame length for SBUS is slightly less than 3 ms, that of CRSF is 0.7ms, which is in turn dictated by 100k vs. 420k baudrate, 2.97ms × 100/420 = 0.7ms.

The R9M-Lite is flash with FCC firmware.

Results

Here is the average latency:

  • R9M-Lite => SBUS – 14.07ms
  • Crossfire Micro TX => CRSF – 13.9ms
  • Crossfire Micro TX => SBUS – 20.23ms

The R9M-Lite actually performed surprisingly well, very close to the Crossfire in terms of latency. But note that SBUS protocol has a longer frame length than CRSF protocol – over 2ms.

Another surprising observation is that the Crossfire doesn’t seem to work as well with SBUS, there is an 8ms extra delay compared to when using CRSF protocol on the receiver. Our guess is that the Crossfire goes into 50Hz mode when SBUS is selected as output (instead of 150Hz). However R9M has 150Hz for SBUS.

Here is the full result:

Latency (ms) Mean Stddev Min Max 25% 50% 75% 95% Refresh rate (Hz) Notes
R9M Lite  – SBUS (R9 Mini) 14.066 2.501 7.515 20.205 12.140 14.524 16.016 17.843 150 add 2.97 ms for SBUS
XFIRE Micro – CRSF (Micro V2) 13.900 2.448 8.432 19.635 12.075 13.879 15.752 17.842 150 add 0.7 ms for CRSF
XFIRE Micro – SBUS (Micro V2) 20.231 6.233 7.680 32.831 14.828 20.606 25.346 29.756 50 add 2.97 ms for SBUS
OpenTX latency (switch) 5.626 1.478 2.046 8.937 4.628 5.630 6.574 7.558

All captures for Saleae as well as Python script used to produce the results will be posted on Andrey’s GitHub.

Edit History

  • Jul 2018 – Article published
  • Jan 2019 – Added overview of R9M-Lite and how to setup

17 thoughts on “Review: R9M-Lite Module & Latency Testing vs TBS Crossfire

  1. Bozos

    Nice comparison. Full size r9m has more latency? For not so long range but freestyle flying and racing the r9m lite is better than the full size one?

    Reply
  2. Eli

    ok, R9mLite is good if we speak about latency but r9m lite has less output power than it told in specs.
    one of the tests on youtube got only 60 m-watts instead of 100.
    another one disappointing thing is that r9m lite could not be moded to get more than 100 m-watts like crossfire up to 250.
    but I dont think r9m-lite is bad transciver. it gives you possibility to fly over the obstacles with a little more range as 2.4ghz but without obstacles.

    Reply
  3. Thomas

    Hi,
    are there two different hardware versions of the R9M lite module (LBT / FCC)? Or is the FCC module only differentiated from the LBT module by the software?

    Reply
  4. Greg Toews

    Found the answer: Testing Methods and Full Result
    For the sake of testing, Andrey built a custom version of OpenTX which doesn’t do this 6-sample filtering. He was testing with the built-in iXJT module in the X-Lite, and an XSR receiver with the latest EU LBT firmware, in D16 8ch mode with telemetry on.

    Reply
    1. Yongwoong Lee

      Thanks for checking Greg!
      But I’m still wonder can I use it more than 8 channels without (a bit) latency penalty :)

      Reply
  5. sevet

    Great comparison!! :)
    Can you please check the R9M lite latency VS the Crossfire
    But with Fport on the R9M lite?

    It is said that Fport is 3-4ms less latency on X4R here:
    youtube.com/watch?v=hCX2Qi51ZtU

    Reply
    1. Andrey M.

      The only reason that is this way is because FPORT firmware is much newer than ordinary SBUS firmware for X4R.
      There won’t be any difference for R9.

      Reply
  6. Don Olsen

    Similar question would this suggest the R9M would perform as well as the R9M-Lite. I’m interested because if so I believe the trainer port on my Spektrum DX6 is SBUS and this may be the solution I need. Instead of either buying a new DX9 $450 & CRSF $225 in order to get 900Mhz but keep the high 150hz refresh for the lower latency.

    Reply
    1. John Hendry

      No that’s not the ticket… DX9 can be moded very easy to use CRSF but without telemetry function, and Spektrum trainer port is PPM so added latency is in effect. I’m waiting on the i12 to mature for XFire.

      Reply

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